1. Field of the Invention The present invention relates to a fuel containing a lubricity additive for improving the lubricating properties of fuels, regardless of whether diesel fuel or jet fuel is involved, and more particularly of diesel fuels with a low sulphur content.
2. Description of the Background
It is well known that diesel fuels and jet fuels must be capable of lubrication, for the protection of pumps, injection systems and of all the moving parts with which these products come into contact in an internal combustion engine. With the intention of employing products which are increasingly pure and nonpolluting, especially devoid of sulphur, the refining industry has been led increasingly to improve its treatment processes for the removal of sulphur compounds. However, it was noticed that, when losing the sulphur compounds, the aromatic and polar compounds, often associated, were also being lost, and this resulted in a loss of the lubricating power of these fuels. Thus, beyond certain contents, the elimination of sulphur compounds from the composition of these products very substantially promotes the phenomena of wear and of failure of moving components where pumps and injection systems are concerned. As in many countries the regulations have imposed a limitation on the acceptable upper content of sulphur compounds in fuels to 0.05% by weight, in order to lower the emissions of polluting combustion gases from cars, lorries and buses, especially in urban built-up areas, these lubricating compounds must be replaced with other compounds which are nonpolluting with regard to the environment but exhibit a sufficient lubricating power to avoid the risks of wear.
A number of types of additives have already been proposed in order to solve this problem. Antiwear additives have thus been added to diesel fuels, some of these being known in the field of lubricants, of the type of fatty acid esters and of unsaturated fatty acid dimers, aliphatic amines, esters of fatty acids and of diethanolamine and long-chain aliphatic monocarboxylic acids, as described in U.S. Pat. Nos. 2,252,889, 4,185,594, 4,204,481, 4,208,190 and 4,428,182. Most of these additives exhibit a sufficient lubricating power, but in concentrations which are much too high, and this is economically highly disadvantageous for purchase. Moreover, additives containing acid dimers, like those containing acid trimers, cannot be employed in fuels fed to vehicles in which the fuel may be in contact with the lubricating oil, because these acids form, by chemical reaction, deposits which are sometimes insoluble in the oil but, above all, incompatible with the detergents usually employed.
U.S. Pat. No. 4,609,376 recommends the use of anti-wear additives obtained from esters of mono- and polycarboxylic acids and polyhydroxylated alcohols in fuels containing alcohols in their composition.
U.S. Pat. No. 2,686,713 recommends the introduction of tall oil up to 60 ppm in diesel fuels in order to prevent rust formation on metal surfaces in contact with these fuels.
Another chosen route is to introduce vegetable oil esters or vegetable oils themselves into these fuels, to improve their lubricating power or their lubricity. These include esters derived from rapeseed, linseed, soya and sunflower oils or the oils themselves (see patents EP 635,558 and EP 605,857). One of the major disadvantages of these esters is their low lubricating power at a concentration lower than 0.5% by weight in the fuels.
To improve the lubricating power of diesel fuels, patent application WO 95/33805 recommends the introduction of a cold-resistance additive consisting of nitrogenous additives containing one or more  greater than Nxe2x80x94R13 groups in which R13 contains from 12 to 24 carbon atoms, is linear, slightly branched or alicyclic and aromatic, it being possible for the nitrogenous group to be linked via CO or CO2 and to form amine carboxylates or amides.
The present invention aims to solve the problems encountered with the additives proposed by the prior art, that is to say to improve the lubricating power of the desulphurized and dearomatized fuels, while they remain compatible with the other additives, especially detergents, and the lubricating oils, especially in not forming deposits and in reducing the cost, especially owing to a lower additive content, markedly lower than 0.5%.
The subject-matter of the present invention is a diesel engine fuel with a sulphur content lower than 500 ppm, including a major portion of at least one middle distillate originating from a direct distillation cut of crude oil, at temperatures of between 150 and 400xc2x0 C. and a minor portion of a lubricity additive containing monocarboxylic and polycyclic acids, the said fuel being characterized in that it contains at least 20 ppm of the additive consisting of a combination of at least one saturated or unsaturated, monocarboxylic aliphatic hydrocarbon with a linear chain of between 12 and 24 carbon atoms, and of at least one polycyclic hydrocarbon compound containing at least two rings, each formed by 5 to 6 atoms of which at most one is optionally a heteroatom such as nitrogen or oxygen and the others are carbon atoms, these two rings additionally having two, preferably vicinal, carbon atoms in common, these said rings being saturated or unsaturated, unsubstituted or substituted by at least one single group chosen from the group made up of carboxylic, amine carboxylate, ester and nitrile groups, the fuel containing more than 60 ppm of additive when the said combination is tall oil.
It has been noticed that the lubricating power introduced by the lubricity additive containing such a combination is well superior to that foreseeable on adding the lubricating powers of each of its components taken separately. This unforeseeable result expresses the synergistic effect of the various components of the said composition with regard to lubrication.
According to a first embodiment of the fuel according to the invention the polycyclic hydrocarbon compound of the said combination is a hydrocarbon compound of formula (I) below: 
with X denoting the atoms of each ring corresponding to 4 carbons, or 3 carbons and a heteroatom such as nitrogen or oxygen, with R1, R2, R3 and R4, which are identical or different, denoting either a hydrogen atom or hydrocarbon groups, each connected to at least one atom of one of the two rings, these hydrocarbon groups being chosen from alkyl groups consisting of 1 to 5 carbon atoms, aryl groups, hydrocarbon rings of 5 to 6 atoms, optionally containing a heteroatom such as oxygen or nitrogen, each ring being formed by direct connection of two groups Ri chosen from R1, R2, R3 and R4, optionally via a heteroatom, the said ring being saturated or unsaturated, unsubstituted or substituted by an optionally olefinic, aliphatic radical containing from 1 to 4 carbon atoms, and Z is chosen from the group consisting of carboxylic groups, amine carboxylates, esters and nitrites.
In a particular version of this first embodiment, the compound of formula (I) is chosen from the group consisting of the natural resin-based acids obtained from residues of distillation of natural oils extracted from resinous trees, especially resinous conifers, and the amine carboxylates, esters and nitriles of these acids.
Among the resin-based acids preference is given to abietic acid, dihydroabietic acid, tetrahydroabietic acid, dehydroabietic acid, neoabietic acid, pimaric acid, laevopimaric acid and parastrinic acid and their derivatives.
In a second embodiment of the invention, the polycyclic hydrocarbon compound of the said combination is a hydrocarbon compound of formula (II) below; 
in which at most one X of each ring is a heteroatom such as nitrogen or oxygen, the other Xs being carbon atoms, in which R1, R2, R3 and R4, which are identical or different, are either a hydrogen atom or hydrocarbon groups, each connected to at least one atom of one of the two rings, these hydrocarbon groups being chosen from alkyl groups containing from 1 to 5 atoms, aryl groups, hydrocarbon rings of 5 to 6 atoms, optionally containing a heteroatom such as oxygen or nitrogen, each ring being formed by direct connection of two groups Ri chosen from R1, R2, R3 and R4, optionally via a heteroatom, the said ring being saturated or unsaturated, unsubstituted or substituted by an optionally olefinic aliphatic radical containing from 1 to 4 carbon atoms, and Z, connected to at least one atom of at least one of the two rings, is chosen from the group consisting of carboxylic groups, amine carboxylates, esters and nitrites.
According to the invention the monocarboxylic aliphatic hydrocarbon is in the form of acid, of amine carboxylate and of esters.
In a more advanced version of the invention, the combination includes from 1 to 50% by weight of at least one compound corresponding to at least one of the formulae (I) and (II) and from 50 to 99% by weight of at least one saturated or unsaturated, linear monocarboxylic acid containing from 12 to 24 carbon atoms, these products being present in the form of acid, of amine carboxylate or of esters.
Amine carboxylates are intended to mean compounds resulting from the reaction of these acids with primary, secondary and tertiary amines or polyamines containing from 1 to 8 carbon atoms per chain and primary, secondary or tertiary alkyleneamines and alkylenepolyamines containing from 2 to 8 carbon atoms. In a preferred version of the invention these amine salts are derived from mines chosen from the group consisting of 2-ethyl-hexylamine, N,N-dibutylamine, ethylenediamine, diethylenetriamine and tetraethylenepentamine.
Among the esters preference is given to esters of primary alkanols containing from 1 to 8 carbon atoms or else polyalcohols of the group consisting of ethylene glycol, propylene glycol, glycerol, trimethylolpropane, pentaerythritol, diethanolamine, triethanolamine and their derivatives.
In a preferred version of the invention the fuel contains from 50 to 1000 ppm of the lubricity additive.
According to the present invention at least one additive from the group of the additives usually added to such fuels may be added to the said fuel, such as detergent additives, additives which improve the cetane number, deemulsifying additives, anticorrosion additives, additives which improve resistance to cold and odour-modifying additives.
To clarify the advantages of the present invention in comparison with the prior art, examples are given below by way of illustration but without limiting the scope of the invention claimed.